Features of Acute Inflammation: Cardinal Signs, Vascular Changes, Exudate, Leukocytes and Systemic Response

Learning outcomes

• Explain the five cardinal signs and their mechanisms.
• Recognise the vascular, exudative and cellular features of acute inflammation.
• Describe neutrophil recruitment, phagocytosis and the typical microscopic appearance.
• Differentiate exudate from transudate and identify major exudative patterns.
• Recognise systemic features and laboratory changes, including atypical presentations.
• Use features to identify red-flag inflammation requiring emergency action.

1. The cardinal local signs

Sign	Mechanism	Clinical example
Rubor (redness)	Arteriolar dilation and increased blood flow.	Red cellulitic skin or inflamed conjunctiva.
Calor (heat)	Hyperaemia and local/systemic temperature rise.	Warm, tender joint.
Tumor (swelling)	Protein-rich exudate, endothelial gaps and cellular accumulation.	Oedematous wound or joint effusion.
Dolor (pain)	Bradykinin, prostaglandins, ATP, nerve compression and tissue acidosis.	Pain on palpation or movement.
Functio laesa	Pain, oedema, tissue destruction and altered organ physiology.	Reduced joint movement or impaired breathing.

2. Vascular features

1. Brief vasoconstriction: may occur immediately after injury.
2. Arteriolar vasodilation: histamine, NO and prostaglandins increase flow.
3. Capillary bed recruitment: more surface area is perfused.
4. Increased venular permeability: protein-rich fluid exits vessels.
5. Haemoconcentration and stasis: red cells become concentrated and flow slows.
6. Margination: leukocytes move toward the endothelial surface.

3. Exudate and oedema

3.1 Exudate

Exudate is protein-rich inflammatory fluid with leukocytes and debris. Its high protein content reflects endothelial permeability. Fibrinogen can polymerise into fibrin; neutrophils and bacteria create purulent exudate.

3.2 Transudate

Transudate is protein-poor fluid from increased hydrostatic pressure or reduced plasma oncotic pressure. Heart failure, cirrhosis and nephrotic syndrome cause transudative oedema without primary endothelial inflammation.

Feature	Exudate	Transudate
Protein	High.	Low.
Cells	Often many leukocytes.	Few cells.
Cause	Inflammatory permeability.	Hydrostatic/oncotic imbalance.
Examples	Pneumonia, abscess, inflammatory pleuritis.	Heart-failure pleural effusion, cirrhotic ascites.

4. Cellular features

4.1 Neutrophil predominance

Neutrophils arrive within minutes to hours and dominate early bacterial and necrotic inflammation. They have multilobed nuclei, granular cytoplasm and strong phagocytic/oxidative capacity. A left shift or bandemia suggests increased marrow release.

4.2 Rolling and adhesion

Selectins mediate rolling; integrins binding ICAM-1/VCAM-1 mediate firm adhesion; PECAM-1 supports diapedesis. Defects produce recurrent bacterial infections and poor healing.

4.3 Chemotaxis and activation

C5a, LTB4, IL-8/CXCL8 and bacterial peptides direct leukocytes to the focus. Activation produces degranulation, respiratory burst, NETs and cytokine release.

4.4 Monocyte transition

Monocytes become more prominent after 24–48 hours and differentiate into macrophages. Their persistence supports debris clearance and repair but can drive chronic inflammation and fibrosis if the stimulus continues.

5. Microscopic and gross features

Level	Features
Gross	Redness, warmth, swelling, pus, fibrin, ulceration, haemorrhage or tissue softening.
Light microscopy	Dilated vessels, oedema, endothelial activation, neutrophils, fibrin and tissue necrosis.
Abscess	Central liquefied necrotic debris, neutrophils, bacteria and a peripheral inflammatory wall.
Serosal inflammation	Watery, fibrinous or haemorrhagic exudate on pleura, pericardium or peritoneum.
Airway/lung	Mucosal oedema, mucus, bronchoconstriction or alveolar exudate.

6. Systemic features

• Fever or hypothermia: IL-1, TNF and PGE₂ alter the hypothalamic set point.
• Malaise, anorexia and somnolence: cytokines act on the brain.
• Tachycardia and tachypnoea: increased metabolic demand and compensation for acidosis.
• Leukocytosis: neutrophilia in bacterial disease, lymphocytosis in many viral infections and eosinophilia in allergy/helminths.
• Acute-phase response: CRP, fibrinogen, serum amyloid A and hepcidin rise.
• Sepsis physiology: hypotension, mottling, oliguria, confusion, lactate elevation and organ dysfunction.

7. Laboratory clues and limitations

Test	What it suggests	Limitations
CRP/ESR	Systemic inflammatory activity.	Non-specific; may lag or remain low in immunosuppression.
FBC/differential	Leukocytosis, neutrophilia, lymphocytosis or eosinophilia.	Normal counts do not exclude serious infection.
Lactate	Hypoperfusion or altered metabolism.	Not specific for inflammation or sepsis.
Procalcitonin	May support bacterial systemic infection in selected settings.	Interpret with clinical context; not a stand-alone rule-out test.
Fluid analysis	Exudate, cells, Gram stain and culture.	Prior antibiotics and sampling error can produce false negatives.

8. Features that signal dangerous inflammation

• Airway swelling, stridor, wheeze or rapidly falling oxygen saturation.
• Hypotension, altered mental state, mottled skin, oliguria or rising lactate.
• Rapidly expanding erythema, pain out of proportion, bullae, skin anaesthesia or crepitus.
• Severe headache, focal neurology, meningism or reduced consciousness.
• Rigid abdomen, peritonism, severe chest pain or new ECG changes.
• Hyperkalaemia, acidosis or dark urine after crush injury.

9. Outcomes reflected in features

Outcome	Typical feature
Resolution	Signs fade, neutrophils clear, oedema drains and function returns.
Abscess	Persistent focal pain, fluctuant mass, pus and fever.
Fibrosis	Firm scar, stricture, adhesions or reduced organ compliance.
Chronic inflammation	Long duration, weight loss, anaemia, persistent inflammatory markers and tissue remodelling.
Systemic failure	Shock, hypoxia, coagulopathy, renal injury or encephalopathy.

10. Clinical examples

Cellulitis

Red, warm, tender, swollen skin with advancing margins represents local acute inflammation. Look for abscess, lymphangitis, necrotising infection and systemic toxicity.

Pneumonia

Alveolar exudate causes crackles, hypoxia and consolidation. The inflammatory response may be protective or cause ARDS and sepsis.

Anaphylaxis

Rapid mediator release produces urticaria, angioedema, bronchospasm and vasodilation. IM adrenaline is time-critical.

Acute gout

Crystal-triggered neutrophilic synovitis causes exquisitely painful swelling. Infection must be excluded in a hot joint.

11. Quick self-test

1. What causes redness and heat?
   Answer: Arteriolar vasodilation and hyperaemia.
2. What causes inflammatory swelling?
   Answer: Protein-rich exudate, oedema and cellular accumulation.
3. What is the predominant cell in early acute inflammation?
   Answer: Neutrophils.
4. What is the difference between exudate and transudate?
   Answer: Exudate is protein-rich due to inflammation; transudate is protein-poor due to hydrostatic/oncotic imbalance.
5. Name three red flags for dangerous inflammation.
   Answer: Airway compromise, shock, altered consciousness, rapid tissue spread, severe hypoxia, crepitus or organ dysfunction.
6. Why can a normal white count be misleading?
   Answer: Early disease, immunosuppression, marrow failure or overwhelming sepsis may blunt leukocytosis.

12. Take-home summary

Selected references

• NCBI Bookshelf: Acute Inflammation
• NCBI Bookshelf: Inflammation and Repair
• NCBI Bookshelf: Sepsis and Septic Shock

Educational note: This resource supports learning and clinical reasoning. Current local protocols, senior supervision and national guidelines take precedence in patient care.